Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.
A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image. Furthermore, the images printed onto an inkjet receiver are susceptible to defects at short time intervals, from immediately after printing to several minutes while the inks are drying. Finally, the durability of the dried image is also subject to environmental factors such as temperature and humidity which, under certain circumstances, can degrade image durability.
To this extent, pigmented inks have been formulated with various polymers, dispersants and other addenda to provide durable images that can withstand post printing physical abuse and environmental conditions. Pigmented inks for inkjet printing have been formulated with acrylic polymers, however, the acrylic polymers alone are insufficient in providing durable images that resist scratches and other forms of physical abuse.
A second class of polymers that have been used as abrasion resistance additives in pigment-based inks are the polyurethanes, or urethane resins as they are sometimes called. U.S. Pat. No. 6,136,890 discloses a pigment-based inkjet ink wherein the pigment particles are stabilized by a polyurethane dispersant. U.S. Patent Application 2004/0242726 discloses a pigment dispersed by a cross-linking step between a resin having a urethane bond and a second water-soluble polymer.
Although polyurethanes are known for their excellent abrasion resistance, they also have a number of drawbacks. For example, not all polyurethane polymers are conducive to jetting from a thermal inkjet head. In particular, water-dispersible polyurethane particles, such as those disclosed in U.S. Pat. Nos. 6,533,408, 6,268,101, Statutory Invention Registration No. U.S. H2113H, and published U.S. patent applications 2004/0130608 and 2004/0229976 are particularly difficult to jet from a thermal inkjet printhead at high firing frequencies.
It is also known in the art of pigment-based inkjet inks to combine a polyurethane with a second polymer, such as an acrylic polymer or polyester. U.S. Pat. No. 6,794,425 discloses a mixture of a hydrophilic polyurethane and a hydrophobic polymer where the molecular weights of polymers are specified. United States Patent Publication Number 2003/0166742 discloses the combination of a polyurethane and a second copolymer where the acid number of the polymers are specified. The acid number of the polyurethane also creates limitations for use in an inkjet printing system. If the acid number of the polyurethane is too high the resulting abrasion resistance of the image can become degraded, especially under conditions of high temperature and high humidity. If the acid number of the polyurethane is too low a substantial amount of particulate polymer will exist and jetability can become degraded.
Colorless or clear inks, which are substantially free of colorants, are also known in the art of inkjet printing. To this end, the use of a colorless ink supplied in a printhead of an inkjet printer has become increasingly popular. The printhead containing the colorless ink is typically part of the same carriage assembly containing colored inks, and the printer is instructed to jet the colorless ink either simultaneously with or, after the colored inks are jetted. U.S. Pat. Nos. 6,428,157 and 6,206,586 describe an inkjet printing apparatus for applying a composition capable of forming a continuous protective overcoat film.
The colorless inks, also known as overcoat solutions or clear ink compositions, are typically formulated with polymer, water, and other components commonly used in aqueous-based inkjet ink formulations, for example, humectants, organic solvents, surfactants and biocides. U.S. Pat. Nos. 6,723,784, 6,604,819 and 6,543,888 describe a coating liquid and image recording method that provides a transparent topcoat for recordings. Jetting an aqueous suspension of fine polymer particles onto a recorded image forms the topcoat. United States Patent Publication numbers 2006/0100306 and 2006/0100308 disclose the use of polyurethanes and mixtures of polyurethanes and acrylic polymers having specified acid numbers for use in colorless ink compositions.
The colorless inks, also known as overcoat solutions or clear ink compositions, are typically formulated with polymer, water, and other components commonly used in aqueous-based inkjet ink formulations, for example, humectants, organic solvents, surfactants and biocides. U.S. Pat. Nos. 6,723,784, 6,604,819 and 6,543,888 describe a coating liquid and image recording method that provides a transparent topcoat for recordings. Jetting an aqueous suspension of fine polymer particles onto a recorded image forms the topcoat. United States Patent Publication numbers 2006/0100306 and 2006/00100308 disclose the use of polyurethanes and mixtures of polyurethanes and acrylic polymers having specified acid numbers for use in colorless ink compositions.
Both pigment and colorless inks can be difficult to jet through inkjet print heads having small nozzle diameters especially by the thermal inkjet printing process. In recent years, thermal inkjet printers have moved to higher jetting frequencies to provide faster printing speeds. Thermal inkjet printers are now capable of printing at jetting frequencies in excess of 10 kHz and the need for higher velocity firings is a highly desirable feature. However, this high frequency firing often comes at the cost of variability in the firing frequency which leads to poor image quality in the final printed image. In addition, the demands of current thermal inkjet printing requires that the nozzles fire for a large number of firings during the life-time of a printer. As an example, a typical inkjet nozzle may be required to fire in excess of 5×107, and up to as many as 1×109, individual firing events without malfunctioning or ceasing to fire altogether.